The present invention relates to a wet developer for use in an image-forming apparatus using a wet electrophotographic system, such as a copying machine, a printer and a digital printing machine.
In an image-forming process of the electrophotographic system, in general, an electrostatic latent image is formed on an electrostatic latent-image supporting member such as a photosensitive member, for example, by exposing an image in association with a document image or image data thereto, and by developing this electrostatic latent image, a visible toner image is formed, and the toner image is transferred and fixed onto an image-recording medium so that a target image is obtained.
The developing system can be classified into a dry developing method and a wet developing method. In the dry developing method, a toner or a toner to which a carrier having a magnetic property or the like is added is used as the developer. In general, a toner, mainly composed of a pigment and a binder resin, to which a charge control agent, a conductivity control agent, a plasticizer, a release agent and the like are externally or internally added, if necessary, is used as a dry toner. In contrast, in the wet developing method, a wet developer, prepared by dispersing toner particles mainly composed of a pigment and a binder resin, a charge control agent, a dispersion stabilizer and the like in a dispersion medium (carrier solution) having an electric insulating property, is used. The toner particles to be used in the wet developer can be formed into fine particles because there is no fear that the particles might be scattered and released into the air, and those having an average particle size of sub-micron can be put into practical use. For this reason, the resulting advantages are that an image having high resolution can be obtained, superior gradation can be achieved, and the like. For example, Patent Document 1 (Japanese Patent Application Laid-Open Publication No. 10-282733) describes a wet developer containing a polyester resin as the binder resin, colorant fine particles having a volume-average particle size in a range from 1.5 to 5.0 μm and an electrically insulating solvent in which the colorant fine particles are dispersed.
Moreover, the wet developer, which is stored in a developer container, is applied onto a developer supporting member such as a developing roller, and allowed to develop an electrostatic latent image on an electrostatic latent-image supporting member, and the image is transferred onto a recording material through an intermediate transfer member, if necessary. It is necessary that the wet developer achieve both of transferring property and handling property. For this reason, for example, Patent Document 2 (Japanese Patent Application Laid-Open Publication No. 2001-100533) describes a method using a wet developer having a thixotropic property.